A large variety of positioning or location systems capable of tracking persons or mobile objects in real-time, e.g. within a work place or a factory exist. Such systems determine a position of a portable tag attached to the person or object by establishing a communication between the portable tag and one or more stationary US stations. Many different methods and types of signals have been suggested for such systems.
Radio Frequency (RF) signals can be used to determine the position of the portable tag, e.g. by calculating the geometric position of the portable tag based on triangulation, i.e. based on sensing phase differences in an RF signal from the portable tag received at three stationary RF stations. Alternatively RF systems may estimate position based on amplitude of the RF signal. Such systems only require rather few RF stations, since RF signals can cover a large area, since such signals easily penetrate building elements etc. However, for zone location applications, RF signals are less suited, since the geometric position of the portable tag must be determined rather precisely e.g. to be able to determine in which of two neighbouring rooms the portable tag is present. An error of 30-40 cm is enough to make confusion between such two rooms, if a tag is positioned close to the wall separating the two rooms.
Infrared signals (IR) have also been suggested for positioning systems. Such systems are suited for zone location, since IR signal radiation is naturally limited by floor, walls and ceiling of a room. However, in practice IR sensing is vulnerable and requires many sensors to cover one zone, since IR signal communication is easily hindered if there is not a clear line-of-sight between transmitter and receiver, e.g. a thin curtain may be provide enough obstacle effect to destroy IR communication.
Ultrasound (US) signals are suited for zone location system due to the inherent property that US signal radiation is limited by floor, walls and ceiling of a room, thus providing automatically an intuitive split into zones. Compared to IR signals, US signals are less vulnerable to obstacle effects, since many obstacles will provide acoustic reflections of US signals, thus serving to provide an acceptable US connection between a receiver and a transmitter even if there is no clear line-of-sight. However, compared to RF signals, location determination by means of US signals requires US stations positioned in all zones, and in zones defined as one large room or two or more interconnected rooms, even more US stations are required per zone. Further, location systems based on US signals have a rather poor capacity. This is due to the fact that US signals travel at the speed of sound which is much slower than RF waves, and further, the bandwidth of US waves is limited compared to RF signals. As a consequence, the required US signals have a considerable longer temporal extension, such that determination of location of one single portable tag may take one second or more. This is too long for the systems to function properly e.g. if 20 or 30 tags are located in one zone at the same time.
Location systems utilizing a combination of US and RF signals have been suggested. The somewhat limited spatial coverage of US based systems have been overcome by providing a US location method which is supplemented by an RF location method to be used in cases where the US location method fails due to a limited spatial range, e.g. where a person wearing a tag has moved outside the predetermined zones.
US 2005/0140508 A1 discloses a location system adapted to receive an RF signal and an IR or US signal associated with each other, thereby allowing unique data regarding both signals to be provided in just the RF signal, thereby saving power for sending IR or US signals.
US 2006/0077759 A1, by the inventor of the present invention, discloses an US location system in which identification tags are equipped with an US receiver and an RF transmitter. The identification tags receive an US signal whose arrival time they measure. This arrival time together with an identification code is sent by the tag in an RF signal to a central unit which then calculates the position of the identification tag. Battery life of the tag is improved, since it does not need to send US signals but only an RF signals which are less power consuming. However, the US arrival time measurement is complex and noise-sensitive, and thus in practice often reduces range to less than desired Therefore, the robustness and usefulness of such a system is rather limited.
WO 2007/110626 describes a method for determining a 3D position of a mobile component relative to a fixed component. The fixed component has a plurality of fixed US transducers in a spaced-apart arrangement and an RF transducer. The mobile component has an US transducer and an RF transducer. The fixed component sends an RF trigger signal, causing the mobile component to send a US signal and resetting timers associated with each of the fixed US transducers. US arrival times at each of the plurality of fixed US transducers are then detected, and trigonometric calculations are used to determine the 3D position of the mobile component. Such system is capable of high precision 3D positioning, but for zone location purposes the distance range and capacity is far too limited. Further, the mobile component requires a power consuming US transmitter.
The paper “WALRUS: Wireless Acoustic Location with Room-Level Resolution using Ultrasound” by G. Borriello, A. Liu, T. Offer, C. Palistrant and R. Sharp, in Proc. 3rd Int. Conf. on Mobile Systems, Applications, and Services (Seattle, Wash., June, 2005). ACM, New York, N.Y., describes an US based location system suited to determine in which room a mobile unit is present. The mobile unit transmits an RF signal to a central station.
US 2002/0167417 A1 describes a zone location system where one US transmitter is placed in each zone and transmits a unique code in the US signal, thus identifying the zone by US. A portable tag receives the US signals and transmits in an RF signal the unique code identifying the zone together with its own identification code.
However, both of the systems described in the “WALRUS” paper and in US 2002/0167417 A1 suffer from a rather coarse location precision which is inconvenient for covering large rooms or rooms with complicated geometry.